Froth flotation is a widely used process for separating fine solids from other solids by taking advantage of the disparity in wetability at solid particle-surfaces. Separation of a solid mixture may be accomplished by the selective attachment of hydrophobic solid particles to gas bubbles. Most often the gas used is air, which is generally passed through a liquid mixture of the crude solids at such a rate as to provide a sustained “froth” or accumulation of bubbles at the liquid-surface interface. The density difference between the gas bubbles and liquid provides the attached solid particles with buoyancy, lifting the hydrophobic solid particles to the surface and leaving behind non-hydrophobic solids in the bulk liquid mixture. The hydrophobic solid particles at the surface remain attached to the surface froth and can be subsequently separated from the bulk mixture by draining the bulk mixture or mechanically skimming the surface froth.
In froth flotation a frothing or foaming agent is added to stabilise the bubbles which carry the hydrophobic solid particles to the surface. The stabilisation of the bubbles or surface froth greatly enhances the separating efficiency of the unwetted hydrophobic particles from the bulk liquid-solid mixture. The frothing agent or “frother” acts by stabilising the bubbles through the lowering of the liquid surface tension enhancing the performance of dissolved air froth flotation processes. “Stabilisation” in relation to flotation refers to both the increased lifetime of the bubbles and the increased bubble surface area generated by lowering the average bubble size.
Froth flotation techniques have been used in the mining industry for around 150 years. Many different minerals and non-minerals are processed using froth flotation. These include for example silver, nickel, zinc, titanium, cobalt, and chromium (metals) as well as quartz and kaolinite (non-metals). Today, mining companies are finding it more and more difficult to find high grades of ore. Accordingly, there is a growing demand to provide more effective separation processes to balance the ever increasing economical burden which mining companies face as they process poorer grades of ore. As such, improvements in froth flotation techniques would be of much benefit to the present day mining industry.
Froth flotation has also been used in other industries including the chemical industry. It is also used in sewage and water treatment applications. Examples include the separation of organic contaminants and oil from water streams, de-inking of used newsprint and magazine furnishes in the paper industry as described for example in U.S. Pat. No. 6,544,383 amongst many others, as well as the separation of plastics and so on.
A major user of froth flotation is the coal industry for desulfurisation and for the recovery of “clean coal”. Clean coal is required by end users who set defined specifications for the coal that they purchase. These will usually include maximum limits on ash or “non combustible” matter in the form of clays, gypsum and other minerals, as well as the maximum sulphur content in order to minimise discharge of environmentally unfriendly SOx gases upon combustion of the coal. The process of producing “clean coal” is often referred to as “beneficiation” or coal washing. In coal washing, the mined coal is crushed and slurried in water. A collector, typically a kerosene or diesel hydrocarbon fraction and a frother are added to the slurry and the mixture passed through conventional flotation cells or devices where the fuel rich material is separated from the ash or fuel poor material.
Specially designed flotation devices include for example a Jameson® Cell, Microcel®, and EKOF® cells. Jameson® cells are extensively used in the beneficiation of coal.
A good frother must possess a number of key properties. Different frothers will be better suited to different applications, however, in general terms, a frother must be able to promote the formation of stable air bubbles under aerated conditions. Frothers are typically comprised of both polar and non polar components. The non polar or hydrophobic moiety will orient itself into the air phase while the polar or hydrophilic component will tend to the liquid (usually water) phase. The result is an enhanced bubble wall strength and stability due to localised increase in surface tension. This will enhance the ability of the bubbles to hold and concentrate the desired mineral.
Effective frothers usually contain at least 5 carbon atoms in a straight chain or branched configuration which provide the hydrophobic interaction with the air phase and a polar group which is typically a hydroxyl (—OH) functionality. The balance of these two functionalities will determine the effectiveness of a particular frother for a particular application. Typical frothers currently in use in the mining industry include aliphatic alcohols like 2-ethyl-3-hexanol, cyclic alcohols (for example, pine oil), 1,1,3-triethoxybutane, and polyalkylene glycols.
One of the most commonly used all purpose flotation frothers for coal, base metal and other non-metal flotation is 4-methyl-2-pentanol, commonly known as “MIBC” (methyl isobutylcarbinol). MIBC has long been the reagent of choice in flotation applications which use Jameson® cells.
MIBC displays excellent surface behaviour, solubility and has been widely adopted in the industry due to its relatively low cost and good froth generation performance, especially in applications where the minerals are relatively hydrophobic and readily floatable. However, MIBC is highly flammable displaying a Pensky-Martens closed cup flash point of 41° C. (106° F.). MIBC also omits an unpleasant odour and accordingly is not very pleasant to work with. MIBC is classified as a dangerous good according to the National Standard for the Storage and Handling of Workplace Dangerous Goods [NOHSC; 1015(2001)] and the National Code of Practice: Storage and Handling of Workplace Dangerous Goods [NOHSC: 2017(2001)] and accordingly, requires special care when handled, transported or stored in large volumes. Consequently, this compound poses a substantial occupational, health and safety (OH&S) concern. It is apparent from this that a need exists for a less volatile alternative to MIBC, that has better flammability and odour characteristics and is generally safer to deal with in general use.